Elementary plant Biochemistry and Biotechnology (2+1)

Introduction

• The transfer of genetic information from DNA to RNA molecules and then from RNA to protein molecules accomplishes Gene expression.
  
• RNA molecules are synthesized by using a portion of one strand of DNA as a template in a polymerization reaction that is catalyzed by enzymes called RNA polymerases.
• The process by which RNA molecules are initiated, elongated and terminated is called transcription.
  
• Two aspects of transcription must be considered –
  
1. The enzymology and
2. The signals that determine where on a DNA molecule transcription begins and stops.

Gene

• The term gene was coined by Wilhelm Johansen in 1909 to describe a heritable factor responsible for the transmission and expression of a given biological character.
• In 1911, T. H. Morgan showed that genes were located on chromosomes and were physically linked together and in 1944, O. Avery and his colleagues shown that DNA was the genetic material. Thus, a simple picture of a gene evolved - a length of DNA in a chromosome, which encoded the information for a protein.
• At any given locus, the DNA which is transcribed can be termed a transcription unit.
• In prokaryotes, a transcription unit may consists of several genes (constituting an operon) whereas in eukaryotes, transcription units are almost always equivalent to a single gene.
• In short, Gene (Cistron) is the segment of DNA involved in producing a polypeptide chain; it includes regions preceding and following the coding region (leader and trailer sequence) as well as intervening sequences (introns) between individual coding segments (exons).